When we think about the risks to a business’s viability, or the threats to a nation’s security, we normally ask, “What is the worst that could happen?” Having identified the greatest risks, we can then decide how much effort to spend on reducing or avoiding them. Climate change, surely, should be no different. How else should a head of government decide how much effort to spend on reducing or moderating emissions of greenhouse gases, other than by considering the worst case consequences of failing to do so?

Over the last year, we have worked with scientists and experts in risk from China, India, the UK, and the US, among other countries, to try and understand those worst-case consequences a bit more clearly. More than 40 other scientists, economists, food and agriculture specialists, hydrologists, military strategists, public health, finance and insurance experts, among others, have contributed to our assessments.

To give one example: we all know that as the world becomes warmer, heat waves will become more extreme. The worst-case scenario is that heat waves become so extreme that they are fatal for anyone without air conditioning, even people lying down resting in the shade. With high degrees of warming, this becomes very likely to happen in hotter regions. If the world warms by four degrees, in northern India there is a 30 per cent probability that temperatures will be so high that moderate/heavy outdoor work cannot be carried out in the hottest month. A recent study, by CEEW’s Hem Dholakia, finds that under a worst-case scenario, Delhi, Ahmedabad, Bangalore, Mumbai and Kolkata are projected to experience the highest absolute increases in heat-related mortality this century.

Another example: drought and heat stress on crops. On a high emissions pathway, the incidence of extreme drought affecting cropland could increase by about 50 per cent in South Asia. At its maximum, drought could affect around seven per cent of South Asia’s total cropland in 2050. An assessment by CEEW’s Vaibhav Chaturvedi found that climate change will result in significant economic losses for Indian agriculture: production losses in rice, wheat and maize alone could reach $208 billion in 2050 (in 2010 $ prices) and $366 billion in 2100.

Other extreme risks include river flooding and coastal inundation thanks to sea-level rise. The risks of once-in-30-years flooding could increase six-fold in the Ganga basin, over the course of the century. With one metre of global sea-level rise, the probability of what is now a “100-year flood event” becomes about 40 times more likely in Shanghai, 200 times in New York and 1,000 times in Kolkata.

Heat stress, falling crop yields, water stress, river and coastal flooding are some of the direct risks of a changing climate. The systemic consequences and security risks of such events occurring on a regular basis could be an even greater concern. Shocks to global food productioncan result in food export restrictions and contribute, as has happened in the recent past, to unrest, conflict and state fragility. Extreme water stress, and competition for productive land, could both become sources of conflict between communities. People in resource stressed regions are likely to migrate in huge numbers. Combined with internal displacement of people, even neighbouring and relatively more stable regions would be threatened. The risks of state failure could rise significantly, affecting many countries simultaneously. The expansion of ungoverned territories would, in turn, increase the risks of terrorism. Even technological risks could rise. Growing temptation to take unilateral steps towards climate geoengineering could become a further source of tension and conflict.

How likely are we to reach such high degrees of warming? This depends on two uncertain factors: the future trajectory of global greenhouse gas emissions, and the response of the climate. What we know about both these factors gives us cause for concern. Economic growth, only moderate levels of political effort, and a lack of investment in energy technologies all seem likely to keep global emissions on an upward path for some time. On such a path, high degrees of warming are a substantial risk.

Risk is a function of time, impacts and probability. Our collective shortsightedness often means that we ignore high impact events and outcomes. They might have low probabilities today but their likelihood could increase in non-linear ways over the medium term. Instead, for continuous risk assessments, we would need to identify outcomes we wish to avoid, consider the full range of probabilities and take account of both direct and systemic risks.

Sometimes in the climate change debate, analysis such as this is met with accusations of “alarmism”. We need to recognise how unusual this is. A country’s national security adviser would rarely be criticised for considering the worst case military, intelligence or terrorist threats to the national interest. Similarly, an insurance firm would not be faulted for assessing the worst-case risks to its ability to continue as a going concern – on the contrary, it is obliged by regulation to do exactly that.

We must treat climate change as a threat to our national and global security not because it is a problem the military can solve (it is not), but because a focus solely on what we prefer to happen would be dangerously complacent. We have to plan for the worst, not merely hope for the best. Only if we focused on understanding what is the worst that could happen, would we be able to make well-informed decisions about how hard we should try to avoid it.

The above column was published in the print edition of Business Standard on 21 July 2015.